Here are basics of healthy diabetic diet, you need to get your dad a nutritionist opinion to see how many calories and carbs he can have per meal and snack. Whole grains, fresh fruit and vegetables, lean meat and poultry can be taken. Tell him to avoid white stuff as much as possible like, sugar, white bread, white potatoes, white flour, white rice etc. The less processed and closer to nature the foods the better they are. Whole grain breads, whole grain wheat flour, brown rice and sweet potatoes are few of the healthier choices.